Starter motors are items of mass production which are not used continuously, but only occasionally, that is, when an internal combustion engine is to be started. Consequently, the production of starter motors is frequently not subject to close tolerance manufacturing processes, since the motor should be inexpensive to make. The operating conditions under which the motor has to function require only highly intermittent duty applications, however, a very high degree of reliability. The usual type of starter motor has a shaft on which an armature is positioned, the windings of which are connected to a commutator. In manufacture, it is frequently difficult to compensate for tolerances of the armature assembly--that is, shaft, armature structure and commutator--and particularly the longitudinal extent of the armature assembly in relation to the frame structure of the starter motor.
It has been proposed to compensate for longitudinal play, or tolerances by compensating for such tolerances upon assembly of the entire starter motor. The commutator bearing is formed with a projecting abutment, extending towards the commutator. The length of all the parts and components which influence axial tolerances is measured, and then a compensating disc is selected, the thickness of which permits only the desired longitudinal excursion of the commutator shaft but no more. This overall length which has to be measured is the distance between the commutator bearing and the motor support bearing opposite the commutator, usually an intermediate bearing, which, in turn, is dependent on the length of the housing of the motor. The length of the housing of the motor is subject to tolerances. Additionally, the distance from the abutment surface or edge on the motor housing to the matching abutment at the inside of the commutator bearing must be determined, considering, additionally, such sealing elements as may be provided, as well as the overall length of the shaft of the starter. Depending on the measured amounts, a compensating or spacer disc or washer is then selected which is placed between the commutator bearing and the end of the drive shaft; after this insertion, the starter is completely assembled together.
The foregoing assembly requires substantial measuring steps which, additionally, must be made on the respective parts to be fitted together. Errors in addition may result, and, further, deformation of individual elements which may occur upon assembly may defeat the compensation as calculated, since the final, assembled motor dimensions may deviate from those which have been measured before assembly.
It has already been proposed to form a tapped opening in the end of the shaft of a starter motor adjacent a commutator bearing, and insert a threaded pin therein, which, then, is threaded out of the shaft to provide the required longitudinal play or clearance of the drive shaft, but no more. It is, of course, difficult to maintain the threaded position of this pin in the drive shaft, particularly since the threaded pin, upon operation of the starter motor, can rub against the stationary end shield of the starter motor. Consequently, the threaded pin must be locked against shifting within the commutator bearing. Such locking means may, for example, be adhesive or locking fluids--for example known under the trademark "Loctite".RTM.. In order to insure that so-set threaded pins will not move under dynamic conditions, close quality and supervisory control upon insertion is necessary. It has been found that such securing adhesives or securing compounds, while sufficiently stable for indoor operating conditions, are unstable under the widely varying temperature swings to which automotive vehicles are subjected. Consequently, a threaded pin which was thought to be tight, may operate as a floating screw, and cannot be maintained in its position with sufficient reliability against inadvertent repositioning under vibration conditions.
Adhesives can also be used which, however, have the disadvantage that they will set the screw in such a manner that, under repair conditions, they are difficult to remove so that, upon repair of the motor, for example, e.g. upon re-use of the armature in a new starter housing, readjustment of the end pin can become impossible since the pin cannot be released from the threaded opening due to the binding power of the adhesive.
The usual way of holding a pin in position, by a counter or holding nut, requires an additional element, means to hold the shaft; and the pin upon tightening of the counter-nut and, under widely varying temperature conditions, particularly coupled with vibration, does not provide for a reliable positioning of a threaded pin.